Multistage Linear Frequency Scanning Mass Analysis Processed by Fractional Fourier Transform.

The common method of mass analysis in a linear ion trap mass spectrometer is to eject ions with different mass-to-charge ratios (/) at different times by resonance excitation. Ions are ejected onto an electron multiplier (EM), generating pulsed signals called the ejection signal. Typically, the ejection signal consisting of multiple pulses is smoothed into a wider Gaussian-like pulse due to the limited bandwidth of the electronic system. For different scan rates and different samples, this bandwidth-limited electronic system and signal smoothing process will differentially increase the peak width and thus decrease the mass resolution. In this study, experiments at different scan rates were performed on a "Brick" miniature linear ion trap mass spectrometer. We propose a multistage linear frequency scanning mode that achieves improved mass resolution and extended mass range, in which fractional Fourier transform is applied to process the ejection signals. The proposed signal processing method improves the mass resolution and mitigates the impact of ghost peaks attributed to scattered ion ejection. A peak width of 0.84 Da at a / value of 242 and a scan rate of 8000 Da/s was achieved in experiments.